The ultrastructure of the extraocular motor units will be studied in the adult rat trochlear nucleus, nerve, and superior oblique muscle. Measurements and the pattern of various components of the motor unit will be examined to test the neuron size principle for slow and fast units, and compared to muscle fiber classification. The muscle will also be examined for sensory nerves and multiple motor endplates. The role of polyneuronal innervation during postnatal differentiation, as well as the developing synaptic pattern of the trochlear neurons will be studied. Horseradish peroxidase will be used to evaluate the sensory innervation, the size principle with respect to retrograde axoplasmic flow and the neurotrophic mechanisms involved. The morphological specificity of the neuromuscular components in adult extraocular motor units, their development, and their properties in retrograde transport of tracer molecules will be related to possible control mechanisms of eye movements.